Conventionally, members made of aluminum or aluminum alloy, such as a variety of exterior parts and structural parts including pistons and cylinders of internal combustion engines and hydraulic-pneumatic pistons and cylinders, have been anodized to form an anodized film (alumite) on the surfaces of the members for the purpose of improving corrosion resistance and wear resistance or coloring.
For this anodizing treatment, DC anodizing treatment, in which electrolyzing treatment is performed by applying DC voltage between a workpiece (anode) and a cathode in the state in which the workpiece is immersed in an electrolytic solution, has been primarily used. In the DC anodizing treatment, treatment is generally performed at a current up to about 3 A per 1 dm2 of the surface area of the workpiece to prevent burning.
In such treatment, a film thickness formed per unit time, that is, a film growth rate, is generally low: for example, the film growth rate is not higher than 1.0 μm/min for an expanded material or an AC material, and is not higher than 0.5 μm/min for an ADC material containing 7.5% or more Si. Therefore, depending on the number of workpieces, a time of 20 to 40 minutes has been required for one cycle of treatment. Also, due to the influence of a contained alloy component, a defect is liable to be produced in the film, and there also arises a problem of occurrence of corrosion in the defective part.
In JP 04-198497 A, in addition to current control, the electrolytic solution is concentrated in a portion of the workpiece by means of masking using a special jig to accomplish forced convection and forced cooling, by which a film growth rate of 13 μm/min is realized for an AC material while the burning and melting of film are restrained. However, it is difficult to apply this technique to large parts and intricately shaped parts.
For the above-described DC anodizing treatment, an attempt has been made to perform anodizing by applying AC voltage or superimposed voltage of DC and AC to between the workpiece and the cathode (refer to JP 06-167243 A, JP 57-169099 A, JP 49-023978 B, and JP 62-253797 A). All of these Publications describe only examples at a commercial frequency level. In the electrolyzing treatment in such a low frequency region, the current density is low, and the treatment speed and the treatment quality are not improved greatly.
The present inventors discovered a treatment method for forming a high-quality anodized film at a high speed without being affected by the alloy component by repeating the application of positive voltage for a very short period of time and the removal of charges alternately as disclosed in JP 2006-83467 A. In this method, since the temperature rise is restrained by the removal of charges, film growth rates of 7.5 μm/min for an AC material, 4 μm/min or higher for an ADC material containing 7.5% or more Si, and 2 μm/min or higher for a casting surface are realized, and also, reduction in film defects is achieved.